1. Field of the Invention
The present invention relates to a method of transferring conductive balls onto a work piece.
2. Background Art
In manufacture of an electronic part with bumps such as a flip chip and a BGA (Ball Grid Array), conductive balls such as solder balls are used. A suction head has been used to transfer the conductive balls onto a work piece such as a chip and a substrate.
The suction head is provided at a contact surface thereof with a number of pores communicated with a vacuum source. The suction head vacuum-sucks at the contact surface the conductive balls in a container or the like and picks them up. The suction head is moved to a position above the work piece and lowered to mount the conductive balls onto electrodes of the work piece. Since a number of conductive balls can be simultaneously mounted onto the work piece, an operation efficiency is advantageously excellent.
However, in the mounting of the conductive balls onto the work piece using the suction head, there is a problem that picking up errors or mounting errors often occur.
In order to overcome such errors, counter measures have been proposed in, for example, JP-A-8-12671. In JP-A-8-12671, as shown in FIGS. 2-4, the suction head is provided with a vibration applicator. The suction head is vibrated by the applicator when the conductive balls are picked up by the suction head and mounted onto the electrodes of the work piece. According to this process, redundant conductive balls are forcedly removed from the surface of the suction head.
However, in the prior art described above, a vibration frequency of the vibration applicator is fixed. Therefore, when the vibration is applied to the suction head, if the frequency of the vibration applicator coincides with a natural frequency of the suction head which is determined by the structural properties and the mass of the suction head, resonance occurs and then the suction head can vibrate at a larger amplitude. If not, there is no resonance, and then the suction head vibrates at a normal amplitude. In such a case, it is difficult to remove the redundant conductive balls fully from the surface of the suction head. The picking-up errors and the mounting errors cannot be completely eliminated.